Traditionally, Digital Signal Processors (DSPs) have been used to run single channels, such as, for example, a single DS0 or time division multiplexed (TDM) slot, that handle single services, such as modem, vocoder, or packet processing. Multiple services require multiple channels and multiple DSPs, each running its own small executive program (small kernel) and application. The executive programs reserve some area in memory for application code. When applications need to be switched, these executive programs overlay this memory with the new application.
Channels may take one of the following forms: one channel carried on a physical wire or wireless medium between systems (also referred to as a circuit); time divisional multiplexed (TDM) channels in which signals from several sources such as telephones and computers are merged into a single stream of data and separated by a time interval; and frequency division multiplexed (FDM) channels in which signals from many sources are transmitted over a single cable by modulating each signal on a carrier at different frequencies.
Recent advances in processing capacity now allow a single chip to run multiple channels. With this increase in capacity has come a desire to run different services simultaneously and to switch between services.
A current method to implement multiple services or multiple channels involves writing all control, overlay, and task-switching code for each service or channel. This requirement causes additional engineering overhead for development and debugging of the applications. In addition, not all services may fit into the memory available to the DSP, and the services must be swapped in from the host system. This swapping—overlaying—adds significant complexity to the implementation of the DSP services. The extra development activity consumes DSP application development time.